Heretofore, a technique for improving wear resistance and durability of a printing plate by adding a microgel to a photosensitive resin layer for lithographic, letterpress, intaglio, and flexographic printing plates has been known.
However, in the offset printing plate, the photosensitive resin layer has extremely small thickness such as several microns and the photosensitive layer must be formed by coating a resin composition after dissolving it in an organic solvent. Thus, the following performances are considered to be necessary for a microgel to be used:
(a) The particle size of the microgel is not more than 1 micron (submicron),
(b) The microgel is sufficiently dispersed in an organic solvent without aggregation or precipitation,
(c) The microgel is compatible with a binder resin to homogenize or integrate.
However, it was found that, when a conventional microgel is used as a composition for the production of the offset printing plate, dispersion of particles is not necessarily sufficient and the above performances can not be satisfied.
For example, in U.S. Pat. Nos. 4,550,073 and 4,551,415, a photosensitive composition wherein a microgel synthesized by suspension polymerization is added is disclosed. However, in the stabilization of particles by a suspension stabilizer, a hydrated layer is not sufficiently formed at the particle surface in comparison with the stabilization of particles by an emulsifier in the case of emulsion polymerization. When the stirring force on polymerization is too large, there is a disadvantage that particles are aggregated, and it is unstable. Further, when synthesized particles are used by dispersing them in an organic solvent, there is a disadvantage that the microgel does not show good dispersibility to the organic solvent because the particle surface is coated with the dispersion stabilizer.
Accordingly, when such a microgel is used for the offset printing plate, dispersion stability of microparticles in organic solvent is inferior and, therefore, dispersion stability in the composition becomes inferior, whereby, a good printing plate can not be obtained. Namely, it does not show good printability and durability.
In Japanese Patent Kokai No. 63-8648, there is disclosed a photosensitive flexographic printing plate comprising a microgel. Further, in Japanese Patent Kokai No. 63-17903, there is disclosed a technique for a microgel-containing photocurable resin composition which can be applied for a printing plate wherein polymerization is initiated by irradiation of 400 to 800 nm. In both references, a microgel comprising an ampho-ionic compound as an emulsifier is disclosed.
However, since the ampho-ionic compound has normally poor compatibility with an organic solvent, a microgel synthesized from this compound also does not show good dispersibility to the organic solvent.